1. Field of the Invention
The present invention relates to an optical disk comprised of two base disks fastened together and to an optical disk device for record and reproduction of an optical disk.
2. Description of Related Art
A two layer type optical disk made by fastening two base disks together has two levels of information record layers consisting of information record layers formed on the inner side of each of the base disks.
In other words, in a dual surface, single-layer type optical disk, the surface of a first base disk made from transparent synthetic resin such as polycarbonate, is formed of a pattern which indicates information, then a second base disk formed in the same way as the first base disk, is placed on the first disk with an application of transparent adhesive and the two base disks fastened together by the hardening of this transparent adhesive.
In such cases, the second base disk is formed with a reflective film instead of a semitransparent layer of the first base disk.
In an optical disk configured in this way, when the laser beam for recording and reproduction from the first base disk is for instance beamed so as to focus onto a point on the semitransparent film serving as the information record layer of the base disk, an information signal is reproduced from the information recorded on the information record layer of the first base disk, based on the returning light beam reflected from the semitransparent film.
On the other hand, when the laser beam is beamed so as to focus on the reflective layer serving as the information record layer of the second base disk, an information signal is reproduced from the information recorded on the information record layer of the second base disk, based on the returning light beam reflected from this reflective film.
Accordingly, reproduction of information on the information record layer of the first base disk and the information record layer of the second base disk of the optical disk is performed by using the same optical pickup from one side of the optical disk.
In the known art, optical disks have been proposed with the information record layer of the first base disk and the information record layer of the second base disk having mutually different signal recording densities, and for instance optical disks with the beam input side of the information record layer having a higher record density.
In optical disks of this type, a low density information record layer such as the information record layer of a second base disk in the related art generally uses a laser beam of 780 nm and is also capable for instance of being played back on compact discs (CD) players of the related art. A laser beam on a wavelength of 780 nm thus passes through the information record layer of the first base disk and is reflected from the information record layer of the second base disk and is then detected after again passing through the information record layer of the first base disk and playback (reproduction) of the information record layer of the second base disk thus performed.
The information record layer of the first base disk on the other hand is an information record layer of even higher density and is configured for a laser beam of a shorter wavelength for instance of 650 nm or 635 nm. This laser beam with a wavelength of 650 nm or 635 nm is reflected at the information record layer of the first base disk and playback (reproduction) performed by detecting the returning light.
In recent years, along with the spread of optical disks having high density information record layers for laser beams on wavelengths of 650 nm or 635 nm (hereafter, medium density record layers), optical disks of even higher density have been developed.
These optical disks for even higher density recording use a blue laser light on a wavelength for instance of 410 nm. Reproduction (playback) devices for optical disks having low density recording layers or medium density recording layers cannot reproduce (playback) these high density optical disks.
In view of the above situation, this invention therefore has the above object of providing a two layer or three layer optical disk made by fastening together two base disks and an optical disk device for recording and reproduction of this optical disk, and capable of recording both low density record information and high density record information, or alternatively capable of recording all three types of densities, namely medium density recording information as well as low and high density record information.